While a fall in risk would be positive news, the findings don’t mean that rising temperatures and changing rainfall patterns will be beneficial for Africa overall, say the authors.

And in the meantime, it shouldn’t be a reason not to push on with disease control programmes to tackle malaria now, another scientist tells Carbon Brief.

The female of the species

Of the approximately 3,500 different species of mosquito, around 40 spread malaria. These all belong to the Anopheles family. In West Africa, the most common carrier of malaria is a group of eight species known collectively as Anopheles gambiae sensu lato.

Mosquitoes thrive in the wet and sultry climate of West Africa. They breed in shallow pools of water, laying their eggs in drainage channels, marshes, swamps or even in muddy puddles. Mosquito numbers in West Africa typically peak during the rainy season between June and September.

Female mosquitoes need the protein in blood to grow their eggs. Most Anopheles gambiae species prefer human blood over that of other animals, and some feed exclusively on humans. This makes them prolific at spreading malaria.

A mosquito picks up the malaria parasite when it feeds on an infected person. After an “incubation period” of around two weeks, that mosquito itself becomes infectious. It can then spread malaria to other people it feeds on, transmitting the parasite in its saliva.

Mosquitoes have a relatively narrow range of conditions in which they can live and breed. This means their survival – and capacity to spread disease – is closely linked to the weather. For example, warmer conditions speed up how quickly new mosquitoes grow and hatch, but temperatures above 35C kill them off. Rain provides water for mosquitoes to lay eggs, but hot weather can mean the water evaporates before their young have hatched.

The new study, published in Nature Climate Change, attempts to take all these variables into account in a model to project the impact of a warming climate on malaria in West Africa. The model also takes into account human factors such as how quickly people build up immunity to the disease.

Rising temperatures, changing rainfall

The researchers ran their model to estimate the change in the spread of malaria in West Africa between the periods of 1975-2005 and 2070-2100. Their projections assumed humans don’t curb greenhouse gas emissions through this century.

Their results suggest western parts of the region will see a decline in annual rainfall by the end of the century (red shading in map below), while eastern parts see an increase (blue shading). All areas are expected to experience temperature rise of around 3-5C.

The researchers break down the results into four different parts of West Africa (as labelled in the map) to represent the slightly different climates in the region.

Projected changes in wet season rainfall (July to September) for West Africa, from 1970-2005 to 2070-99 (as %), under the RCP8.5 emissions scenario. Blue shading indicates increases in rainfall, while red shading shows decreases. The boxes indicate the four parts the region is divided into for the study. The points on the map show the specific locations used in the modelling. Source: Yamana et al. (2016)

Across all of West Africa, temperatures during the wet season already reach, and exceed, the limits of mosquito survival, the researchers note. This means that future temperature rise will lead to fewer mosquitoes surviving, helping to limit malaria outbreaks.

Glossary

RCP8.5: The RCPs (Representative Concentration Pathways) are scenarios of future concentrations of greenhouse gases and other forcings. RCP8.5 is a scenario of “comparatively high greenhouse gas emissions“ brought about by rapid population growth, high energy demand, fossil fuel dominance and an absence of climate change policies. This “business as usual” scenario is the highest of the four RCPs and sees atmospheric CO2 rise to around 935ppm by 2100, equivalent to 1,370ppm once other forcings are included (in CO2e). The likely range of global temperatures by 2100 for RCP8.5 is 4.0-6.1C above pre-industrial levels.

RCP8.5: The RCPs (Representative Concentration Pathways) are scenarios of future concentrations of greenhouse gases and other forcings. RCP8.5 is a scenario of “comparatively high greenhouse gas emissions“ brought about by rapid population growth,… Read More

In eastern parts, the projected increase in rainfall is not expected to cause much change in malaria occurrence. Northerly areas (box ii in the map above) will still be largely too dry for mosquitos, the paper says. While more southerly areas (boxed iii and iv) are already wet enough for mosquitoes, the hotter conditions mean the potential increase in mosquito breeding is offset by lower mosquito survival rates.

In western parts (box i), the findings suggest that hotter and drier conditions will make the area less suitable for malaria breeding and survival. This will lead to a significant decrease in malaria outbreaks, the paper says.

Positive side

So, for West Africa as a whole, warmer conditions and changing rainfall patterns mean malaria risk is likely not to be made worse, and could fall in some areas.

“West Africa is a region that is hyperendemic for malaria – that is, almost everyone living in rural situations is exposed, and a scary proportion of the population contracts the disease. Climate change is generally considered as bad, and clearly it is on global scales. However, on a regional scale, climate change can have both good and bad elements, and this study points out one that is on the positive side.”

However, it’s worth noting that this study is looking many decades into the future, which means efforts are still needed now to reduce the risk that many communities are already experiencing. As Prof Andy Morse, a professor of climate impacts at the University of Liverpool, who also wasn’t involved in the study, explains to Carbon Brief:

“This paper is not saying that malaria will be reduced in the near future due to climate change in West Africa. Thus we must not misinterpret this work as there being no need to continue malaria prevention programmes in region where malaria is – and will remain – a major threat to life and well-being of its residents.”

The study’s lead author, Dr Teresa Yamana, a research scientist at Columbia University, agrees. While the climate determines the potential for malaria outbreaks, human intervention means we can affect whether they actually occur. She tells Carbon Brief:

“Successful malaria control programs can go a long way in reducing or even, in some cases, eliminating malaria transmission. Of course malaria control has to be sustained, as the effects of climate are long term.”

In sub-Saharan Africa, for example, disease controls – such as providing insecticide-treated mosquito nets – have seen the prevalence of malaria halve since 2000.

‘Not good news’

While a reduction in the spread of malaria might be one unintended benefit of human-caused climate change, it doesn’t mean it will be favourable for Africa overall, says co-author Dr Arne Bomblies, an associate professor at the University of Vermont. He tells Carbon Brief:

“Reduction of rainfall is part of the projected climate changes which in part gives rise to our conclusion, and the regions affected by reduced rainfall are some of the most densely populated on the continent where people rely on subsistence farming of pearl millet.”

Farmers in West Africa rely heavily on rainfall, says Bomblies, leaving them highly vulnerable to famine. The news that malaria risk might decrease will “hardly cheer them up,” he says, even in non-famine years:

“In fact, malaria transmission is already so intense in many parts of the Sahel, that a lack of increase is not even really good news. Anything but a large decrease is not good news.“

In addition, while warming might make some of West Africa too hot for mosquitoes, it’s likely that other parts of the continent will become more suitable for malaria, says Yamana.

Cold temperatures slow the incubation period of the malaria parasite inside mosquitoes. This means in cooler areas – such as at higher elevations – mosquitoes tend not to live long enough to become infectious. But a warming climate could change that, Yamana says:

“Warming temperatures in these [cooler] regions could speed up the incubation period enough to allow mosquitoes to transmit the disease.”

And if malaria spreads to new areas, the chances are that local communities won’t have built up any natural immunity to the disease, notes Bomblies. This means outbreaks are likely to cause high levels of illness and death if malaria rates increase.